Some titles that require a quad core CPU simply won't play at all, like Far Cry 4. Others will simply play poorly, and require significant reductions in settings to be playable. There may also be issues with lag and other performance related errors. Much depends on the CPU model, clock speed, whether or not it has hyperthreading and if it's an overclocked chip or not. Some of those things, such as hyperthreading and overclocking may mitigate some of the negative effects, but the end result in most cases will be a bottleneck of varying degrees and other performance related issues, or not working at all.